You could use a simple calc field using Max() of the related dates. If your need is for that date to change every time a new procedure is entered in the related table, then you don't really need an auto-enter, but if you need to store or index this date for some other reason then you could use the same calc in an auto-enter.
1. Create a new Table Occurrence (TO) for the child table (call it LastProcedure).
2. Link the parent table to the new TO with the same relationship as your current one. On the child side of the relationship sort the relationship by your dateofprocedure - make sure the sort is in Descending order.
3. On the patient layout simply place the field "LastProcedure::dateofprocedure" on the layout and it will always display the date from the last procedure.
How does it work? The field on the layout will show data from the 1st record it finds across that relationship. Because you have sorted that relationship on dateofprocedure in Descending order - the first record it finds is the most recent one.
Hope this helps. Chris